The present invention is an improvement on the electrical heating unit and process for making that heating unit described in U.S. Pat. No. 3,500,444 of Mar. 10, 1970, issued to W. K. Hesse et al entitled ELECTRICAL HEATING UNIT WITH AN INSULATING REFRACTORY SUPPORT. Hesse discloses a block containing ceramic fibers in which an electrical heating element is disposed on one surface of the block. The block itself is described as preferably containing high refractory compositions, such as silica or quartz, magnesia, alumina-silica compositions including those alumina-silica compositions containing titania and/or zirconia, and synthetically produced inorganic fibers which exhibit resistance to deterioration at temperatures up to the order of 2000.degree. to 2500.degree. F. are described as suitable. The fibers themselves are more fully described in an article entitled "Critical Evaluation of the Inorganic Fibers" in Product Engineering, Aug. 3, 1964, pages 96-100. Hesse gives an example of the preferred means for producing the electrical heating units as comprising filter molding from a dilute water suspension of approximately 99% water and 1% solids, the solids consisting of approximately 12% binder, 84% inorganic refractory fibers, and 4% coagulant. In practice, a mat is formed by the molding process, and thereafter the mat is dried and sintered to produce the thermal insulating block.
The electrical heating elements of Hesse are generally tubular in shape and are embedded on the surface of the thermal insulating block. Electrical heating elements have also been mounted on the block in various other ways, such as by brackets as disclosed in U.S. Pat. No. 4,299,364 of Peter J. Loniello dated Nov. 10, 1981, by embedding the electrical heating elements directly beneath the surface, as disclosed by Ewald R. Werych in U.S. Pat. No. 4,278,877 entitled ELECTRICAL HEATING UNIT WITH FLATTENED EMBEDDED HEATING COIL dated July 14, 1981, and by embedding the opposite edges of a flat serpentine heating element in the walls of a slot which extends into the thermal insulating block as disclosed in U.S. patent application Ser. No. 06/608,348 of Ludwig Porzky entitled ELECTRICAL HEATING UNIT WITH SERPENTINE HEATING ELEMENT AND METHOD FOR ITS MANUFACTURE, filed May 8, 1984, now U.S. Pat. No. 4,575,619.
In all of the heating units employing molded fiber thermal insulating blocks and electrical heating elements, the lack of strength of the thermal insulating block is a deterrent to mounting the electrical heating element on the block and to maintaining it in its proper position. The lack of strength of the thermal insulating block is a direct result of the low density of the block, Hesse indicating a range from about 4 to about 30 pounds per cubic foot and preferably about 10 to 15 pounds per cubic foot. Higher densities result in binding together increased numbers of fibers to maintain the block integrity, and hence higher strength.
A second factor which affects the strength of molded fiber thermal insulating blocks is the degree of randomness of the orientation of the fibers within the block. The fibers are mixed into a substantially random universe in a suspension or slurry of water, binder and fibers prior to introducing the slurry into a mold. The fiber content by weight is only of the order of 1% of that of the water in the slurry which is introduced into the mold. However, as the water is drawn from the molded mat through a filter plate, the fibers become pressed upon one another and tend to become reoriented, particularly at the surfaces, and lose some randomness.
In the early stages of mat formation in the mold, the spaces formed between fibers, referred to herein as pores, are filled with the liquid component of the slurry and the fibers tend to float in the liquid component, hence making it necessary to remove the liquid component to increase the density of the mat. In later stages of mat formation, gravitational attraction to the liquid component will remove a certain portion of the liquid component through an underlying filter screen, but the surface tension of the liquid component of the slurry on the fibers trapped in the mat prevents a portion of the liquid component from being drained from the mat. Accordingly, failure to remove a significant portion of the liquid component of the slurry from the mat places a restriction upon the density that can be achieved in the mat during the molding process.
The prior art has utilized principally two alternatives to facilitate removal of the liquid component of the slurry from the mat during the molding process for thermal insulating blocks. First, pressure is exerted on the mat by means of a pressure plate, usually by gravitational attraction from above. The weight of the pressure plate compresses the mat against the underlying filter screen, thereby pressurizing the liquid component of the slurry and overcoming the surface tension of the liquid component on the fibers to permit gravity to withdraw a portion of the liquid component from the pores within the filter mat. Removal of the pressure plate will allow the resiliency of the fibers to expand the mat, thereby creating partial voids in the pores of the mat, but the mat will remain partially compressed. The use of a pressure plate increases the density of the filter mat, but it tends to distort the fibers within the filter mat, and when using excessive pressures, breaks down the fibers and tends to produce cracks in the product. When a pressure plate is used, a thick membrane is formed by the fibers on the surface of the filter mat contacted by the pressure plate and the filter screen.
The second alternative comprises the use of vacuum for removing a portion of the liquid component from the filter mat during the molding process. The mold is subjected to a subatmospheric pressure of about 20 inches of mercury to facilitate removal of the liquid component from the block formed during the molding process. The use of vacuum also tends to form cracks in the finished product and forms a membrane on the surfaces of the molded block, but is effective to increase the density of the block. The fibers throughout the mat produced by a vacuum molding process are less randomly oriented than the fibers in the slurry used to form the mat, particularly at the horizontal surfaces. As a result, thermal insulating blocks produced by vacuum molding have more limited strength than desired, and are of lower density than desired.
The strength and durability of molded fiber thermal insulating blocks result from the contacting regions of adjacent fibers within the block. The liquid component of the slurry used to mold the mat contains a binder, as described above, and when the liquid component of the slurry is removed, a portion of the binder remains and adheres to the fibers, thus forming regions for each fiber that are attached to adjacent fibers by a small mass of binder. Subsequently, the mat is heated to evaporate the water within the mat and cause drying of the binder, thereby producing a thermal insulating block by binding contacting fibers together at their regions of contact in a fixed structure.
The water from the liquid component held in the pores of the filter mat on completion of the molding process cannot be mechanically removed and must be removed by evaporation. Accordingly, the filter mat is removed from the mold following the molding process and dried in an oven operating at a temperature above the boiling point of water. Suitable temperatures for drying the filter mat are in the range of 220.degree. F. to 500.degree. F. Sintering of the binder cannot occur until the water portion of the liquid component is evaporated, since the temperature of the binder will be held to the boiling point of water while water is present. Removal of the water as vapor is effectively achieved by the drying process, but at a cost in energy far in excess of the cost required for mechanical removal of the initial water from the filter mat. After removal of the water from the liquid component remaining in the mat, the temperature of the binder will rise to permit drying of the binder. In practice, the dried mat may then be placed in a furnace operating at a temperature sufficient to sinter the binder.
It is thus an object of the present invention to provide a fiber mat which overcomes the difficulties of such mats known to the prior art, and which may be sintered to provide an improved thermal insulating block. More specifically, the objects of this invention are to provide thermal insulating blocks containing inorganic fibers in which the fibers are more randomly oriented than such blocks prior hereto, to provide such thermal insulating blocks of higher density than the thermal insulating blocks prior to the present invention, to provide such thermal insulating blocks of fibers of greater strength than known prior to the present invention, and to provide such blocks at a lower cost than has been possible using prior art processes. In addition, it is an object of he present invention to produce electrical heating units with thermal insulating blocks of improved construction as indicated above.